1. Field of the Invention
The present invention relates to an image display apparatus for forming an image on an image holding material.
2. Description of the Prior Art
As an image display apparatus for displaying image information converted to an electrical signal as a visual image, a CRT (cathode ray tube) display apparatus and a liquid crystal display apparatus have been put into practical use so far.
Among, them, the CRT display apparatus is most generally used and has a high reliability. However, since a resolution power of such an apparatus is not so high, it is impossible to display small characters such as used in a newspaper, magazine and the like, particularly, thin characters having a number of strokes such as KANJI (Japanese characters) or the like with the sizes as they are. Therefore, they are enlarged and displayed for easy interpretation. But, this enlarged indication causes a problem such that the amount of information which is displayed on one screen decreases since the number of characters which can be displayed on one screen is reduced in inverse proportion to the magnification factor. In addition, when the same image is displayed continuously for a long time, the screen could be burned out, causing the display capability to deteriorate. Further, the flickering of screen results in fatigue of the eyes of the operator. Next, the liquid crystal display apparatus is the display device which has been recently put into practical use. However, this apparatus has an insufficient resolution similarly to the CRT display apparatus and also has other problems such that it is difficult to make a large LCD screen and that it will have become expensive.
Consequently, as a third image display apparatus without the above-mentioned drawbacks, the present applicant has already proposed an image display apparatus using an electrophotographic method such as shown in FIGS. 1 and 2.
In the diagrams, a reference numeral 1 denotes a vertical type external apparatus housing; 2 is a display image observation hole which is largely opened and formed in the front plate of the external housing 1; 3 an operation panel arranged on the upper surface of the lower projecting portion of that front plate which is formed by being protruded on this side; 4 a display window glass attached to the observation hole 2; and 5, 6, 7, and 8 indicate four supporting rollers for a photosensitive material like an endless belt. These supporting rollers are individually rotatably arranged through bearings in the manner such that each two rollers are disposed respectively at the upper and lower portions in the external housing 1 and their axes are parallel to one another in the lateral direction of the external housing 1. A numeral 9 is a photosensitive material like an endless belt which is supported around the four rollers 5 to 8.
This photosensitive material 9 like an endless belt (hereinafter, simply referred to as a belt) is, for example, a transparent sheet whose base layer is a polyester sheet or the like. A metal is extremely thinly evaporation deposited on the outer surface of the sheet and conductivity is imparted thereto while substantially keeping the transparency, thereby forming a photosensitive material layer (photo conductive layer) such as CdS or the like on the side of the metal deposited surface. The photosensitive material 9 is therefore flexible as a whole and is supported by the above four rollers such that the side of the photosensitive material layer faces the outside.
The roller 5 among the four rollers 5 to 8 is used as a drive roller serving to transferring the rotational force of a motor M.sub.1 (not shown). At least one of the other three rollers 6 to 8, e.g., the roller 6 acts as a belt tension roller to apply the tension to the wound belt 9. Thus, when the drive roller 5 is counterclockwise rotated by the motor M.sub.1 at the upper portion of FIG. 2, the belt 9 is rotates counterclockwise without slack and slip. In association with this rotation, the outer surface of the belt 9 moves and passes from the bottom to the top at the portion of the observation hole 2 between the rollers 5 and 8.
A numeral 10 indicates an image exposing apparatus of the laser beam scanning method which is disposed in position in the space inside the belt 9. This image exposing apparatus 10 has a semiconductor laser oscillator 11, a polygonal mirror scanner 12, an f..theta. lens (image forming lens) 13, a reflecting mirror 14, and a transparent plate 15. The transparent plate 15 is the transparent flat plate such as glass, plastic or the like having a long lateral width that is disposed by being pressed by a proper pressure so as to be come into contact with the inner surface of the tensioned belt portion between the belt supporting rollers 7 and 8.
In this image exposing apparatus 10, a laser intermittent beam L corresponding to a serial electrical pixel signal which is supplied from an electronic computer, image reading apparatus or the like is generated from the semiconductor laser oscillator 11 locating at the front portion of the drawing toward the rotating polygonal mirror scanner 12 locating at the rear portion of the drawing. The laser beam L entered the scanner 12 is deflected in the direction of width of the belt and passes through the path of the f..theta. lens 13, reflecting mirror 14 and transparent plate 15. Then the laser beam L enters the side of the inner surface of the belt 9 between the rollers 7 and 8 and scans on the belt in the direction of width of the belt. Due to this, the laser beam scan is used as the main scan and the rotational movement of the belt 9 is used as the sub-scan, thereby performing the image exposure by the laser beam from the side of the inner surface of the belt.
A numeral 16 denotes a toner developer disposed on the side of the outer surface of the belt portion between the rollers 7 and 8. In addition, the transparent plate 15 is arranged such that the lower surface is slightly projected downwardly from the common tangential line on the lower sides of the rollers 7 and 8 and is pressed to be come into contact with the inner surface of the belt 9. End faces 15.sub.1 and 15.sub.2 of the plate 15 on the incoming and outgoing sides of the belt are bevelled and rounded for allowing the belt 9 to be smoothly moved in contact relation therewith. Further, this transparent plate 15 prevents the vertical oscillation of the belt 9 which is caused due to the vertical motion of the belt 9 which is rotationally moved and due to the contact of the developer 16 with a developing brush 35 made of magnet, thereby holding the exposure position at a constant location and improving the resolution power.
First, when a display start command is inputted with a key after setting the desired conditions such as an input instruction of image information, designation of the image display position and the like by the operation with the buttons on the operation panel 3, the belt 9 serving as the photosensitive material starts rotating at a predetermined speed. Next, the laser beam scanning exposure of the image information inputted from a reader, disk or the like is started on the side of the inner surface of the belt 9 between the rollers 7 and 8. Since the toner of the developer 16 acts on the outer surface of the belt simultaneously with this exposure, the toner image corresponding to the exposed image is sequentially formed on the outer surface of the belt 9. When the toner image formed on the outer surface of the belt is rotated and moved from the bottom to the top of the observation hole 2 in association with the rotational movement of the belt 9 and reaches the specified position within a range of the hole, the rotation of the belt 9 is once stopped. Thus, the input image is displayed at the portion of the observation hole 2, thereby allowing the image to be observed through the display window glass 4. Next, when the belt 9 is rerotated in response to a command for instructing the rerotation of the belt, the toner image corresponding to the next input image is formed due to the processing similar to that mentioned above and is moved to the hole range position, so that the next image is displayed. When the toner image on the outer surface of the belt upon completion of the display of the image again reaches the section of the toner developer 16 in association with the rotation of the belt, it is cleaned by the developing brush 35 and the toner is removed from the belt. The belt 9 is newly subjected to the exposure simultaneous development immediately after this cleaning operation, so that a new toner image is formed.
In this way, the image information is reproduced and displayed as a toner image, and the image exposure is performed by use of the very thin laser beam L; therefore, it is possible to perform the image display with high resolution whereby small characters and images can be easily observed and clearly displayed. Moreover, since the exposure simultaneous toner developing method is adopted, this makes it possible to relatively cheaply manufacture an image display apparatus of a large screen with an extremely simple arrangement without requiring any corona charging means or any particular cleaning means. Also, it is possible to constitute a display apparatus with high reliability but less failure and less degradation of the photosensitive material.
However, as described above, since the toner image attached to the belt 9 is moved to a proper location of the glass window 4, it is necessary to match the operation timings in each section of the apparatus such as the timing to write the image, the timing to stop the drive motor M.sub.1, etc. in accordance with the rotational position of the belt 9. Due to this, conventionally, as shown in FIG. 3, for instance, one or two chains 39 are suspended at a predetermined position of the side end of the belt 9 or near both side ends of the belt 9 and are driven by the drive roller 5; a light shielding plate 40 is formed at a predetermined location of the chain 39 so as to protrude therefrom; and an operation timing signal is generated when this light shielding plate 40 passes through a groove of a photo interrupter 41 disposed along the moving surface of the belt 9; thereby directly detecting the rotational position of the belt 9 by means of the belt 9 or chain 39.
However, since the belt 9 or chain 39 has a band-like shape, they could cause a vertical fluttering (waving) in the moving direction while they are rotating. Thus, there is a problem such that the light shielding plate 40 hits the photo interrupter 41 due to such fluttering, so that the photo interrupter 41 is easily damaged. In case of directly detecting the rotational position of the belt 9 as the photosensitive material by means of the belt 9 or chain 39, this problem will be always caused even if other detecting means such as a microswitch, hall device or the like is used.
In addition, in such an image display apparatus, another problem occurs when there is a difference in size between the display surface and the display image. For instance, it is assumed that the display image of the vertical A4 size is inputted to the display surface of the vertical A3 size. In the case where the image of the vertical A4 size is displayed using image display means of the vertical A3 size, the image of the vertical. A4 size is displayed at the upper left location on the display surface. In this way, when the sizes of the display surface and display image differ, the display location is one-sided, causing a drawback such that it is extremely difficult for the operator to observe the image.
On the other hand, since the above-described image display apparatus merely displays the image inputted, there is another drawback such that it is difficult to designate the location (or to designate the location when a map and the like are displayed, or the like) when images are editted.
Further, in such a kind of apparatus, the position where the belt stops is not fixed but may be often shifted due to the inertia. This shift amount is accumulated and a joint of the belt appears in the image display surface, so that this may results in an unsightly image.
Also, in such a kind of apparatus, there is a case where the display location of the image information is shifted when the image is displayed, causing a situation such that all of the image information of one page cannot be displayed.
In addition, in such a kind of apparatus, after the power supply was turned on, the image display is started from the stop position of the belt at that time, so that the display location of the image is not constant and the image is displayed over a joint of the belt, causing an unseemly image.
Moreover, in such a type of apparatus, there is a case where an abnormal operation such that the belt does not stop after rotation due to some reason occurs. However, since this kind of apparatus does not have any particular operation sequence, the power supply of the apparatus has to be turned off to solve this abnormal operation.